GB2167703A - Cooling extrusion coated optical cables - Google Patents

Abstract

In the manufacture of optical fibre cable a secondary coating such as a Nylon material is applied by extrusion to the optical fibre by pulling the fibre through an extruder crosshead (12) and along a cooling air path. Obtaining adequate cooling while providing for a reasonable production rate is difficult without needing an excessively long air path. In the present arrangement the above desiderata are achieved by passing the coated fibre through three cooling rings (13, 15, 16) spaced along the fibre's air path. The second ring is further from the first ring than the first ring is from the crosshead, and the third ring is further from the second ring than the second ring is from the first ring. Each cooling ring preferably has a bore of Venturi form, to enhance the air cooling effect. <IMAGE>

Description

SPECIFICATION Optical cable manufacture The present invention relates to optical cable manufacture, and especially to the application to an optical fibre of a secondary coating.

Optical fibres usually consist of a core which is the portion which conveys the light beam, which core is overlayed by a cladding of a somewhat lower refractive index. Such fibres are provided with a secondary coating of a plastics material such as a Nylon, and to apply this coating the fibres are extrusion coated to 1 mm outside diameter (o.d.) or 0.85 mm o.d., using a line speed of 40 mimin. There is a need to go to higher line speeds (100 m/min) to increase the volume of production while keeping overall costs down. This is difficult to achieve when stable secondary coatings are a primary requisite.

In the current 40 mlmin extrusion coating of fibres with Nylon 12, an air path of 5 m is needed to achieve a package stable both with time and temperature. To maintain the same cooling conditions while increasing the line speed to 100 mlmin would require an air path of 12.5 m. Such a long unsupported length is undesirable, primarily to avoid vibrations resulting from line resonances. To avoid such long unsupported lengths, the philosophy adopted was to reach the fastest crystallisation temperature quickly.

To justify this approach, consider the variation in the Nylon coating temperature with time, i.e. from the moment it is extruded on to a fibre at about 205 C, until it cools down to room temperature (200C). Figure 1 is a graph of the Nylon surface temperature with time for an extrusion speed of 40 m/min. A plateau A B due to crystallisation is apparent, which is confirmed by rapid change in opacity. At high speed, the extrudate must go through the complete plateau A-B, to provide the right level of compressive strain, so ideally quenching should be at a point C. Hence one can force cool the extrudate along the path D-A in as short a time as possible to reduce total air path for cooling.If quenching is applied earlier than the point C, so that it encroaches on the crystallisation plateau, then amorphous coating with a density of about 1006 kg/m3 is obtained, compared to 1013 kg/m3 for quenching at point C.

An object of the invention is to provide a method of applying an extrudate rapidly to an optical fibre to produce a secondary coating thereon which enables the higher speeds desired to be attained without the disadvantages indicated above.

According to the invention there is provided a method of applying a secondary coating to an optical fibre, in which the fibre to be coated is passed through an extrudate cross-head for the application of the secondary coating to the fibre, whereafter the coated fibre is passed along an air path to cool the secondary coating, and in which during the passage of said coated fibre along the air path it passes through a plurality of cooling rings each of which directs a cooling air flow onto the coated fibre so that the air directed onto the coated fibre by each said cooling ring embraces the coated fibre and thereby exerts a cooling influence on the secondary coating.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Figure 2 is a cross-section of a cooling ring such as may be used in the present method.

Figure 3 is a schematic diagram of an extrusion line which embodies the method of this invention.

Figure 4 is a graph similar to that of Figure 1, but for a system of the type shown schematically in Figure 3.

Figure 5 is a cooling ring of the type known as an air amplifier, usable in the system of Figure 3.

Figures 6 and 7 are further graphs explanatory of the system of Figure 3.

The method of rapid cooling of extrudate which we have found to give excellent results is to use a multiple of air-cooling rings suitably placed along the extruder line. Experiments were initially carried out using cooling rings such as shown in Figure 2, with various forms of extruding crossheads.

The cooling ring just mentioned has a central hole 1 through which the extrusion coated fibre passes.

Surrounding this hole there is a chamber 2 fed with air under pressure from an air input 3. This chamber 2 communicates with the hole 1 via a 5 thou air gap indicated at 4. Hence when air is supplied under pressure to the input 3, it passes, due to the internal shape of the chamber to the hole 1, where it acts on a passing fibre. Thus an annular cooling blast of air embracing the fibre whose secondary coating is to be cooled is provided.

We now refer to Figure 3, which shows the extrusion line in its bare essentials. The primary coated fibre is supplied from a braked fibre pay off roll 10, braking approximately 120g, via a constant speed capstan 11 to the extruder crosshead 12. The secondary coated fibre issues forth from the crosshead 12 coated and hot. To cool it it passes along a long air path, whose length is discussed below.

In the present arrangement, during this passage the coated fibre passes in sequence through a first cooling ring 13, a control diameter gauge 14, a second cooling ring 15 and a third cooling ring 16. As will be seen below the main distinguishing feature of the present method is that we use more than one cooling ring in conjunction with the air path.

At the other end of the air path, the coated fibre passes through a water trough 17, an air wire 18, and a monitoring diameter gauge 19 to a constant tension capstan 20, from which it passes to a take-up drum via an accumulator. Most of the integers used in this process are known and so will not be discussed herein.

It was found experimenting that by using two cooling rings of the type shown in Figure 2, placed 50 mm and 260 mm respectively from the crosshead, an air path of 9.0 m gave a coating density of around 1013 kg/m3. However, a comparable density is achieved for a reduced air path of 7.5 m by using a third cooling ring 16 placed 1110 mm from the crosshead.

To achieve an economical manufacturing process at 100 mlmin using as short an air path as possible, it was essential to find the position of the crystallisation plateau. Figure 4 shows the variation in coating surface temperature along the extruder line measured using a Transmet thermometer, and with three air cooling rings in the line. The crystallisation plateau is approximately 2.5 m wide ( 1.55), which compares well with the observed plateau width of 1 m for a line speed of 40 mlmin. This indicates that an air path of 8 m is needed with this cooling ring design to achieve a reasonably consistent and stable coating.

Hence a fourth cooling ring was placed at 1550 mm from the crosshead. With an air path of 7 m, this gave a coating with a density of 1012.9 kglm3. This shows negligible improvement, so that more efficient cooling using three cooling rings is needed. Thus although the use of extra cooling rings of the relatively simple type shown in Figure 2 gives a useful improvement, further improvement is possible.

To achieve more efficient cooling than in the method just described, commercially available "air movers" were used in place of the rings. An air mover is an airflow amplifier - it uses the energy from a small volume of compressed air to produce a high velocity, high volume, low pressure output airflow. By having two or three such devices in series a "wind tunnel" airflow effect is created.

Figure 5 shows the principles of operation of an air mover.

Here compressed air flows from a supply inlet 30 into an annular chamber 31. This supply of air is throtted by an annular gap 32, so that a thin layer of high velocity air adheres to profile 33 which turns the air flow through 900 to pass down the line 34. This air flow, due to the Venturi effect, causes a pressure drop as a result of which large volume of ambient air flow into the line 34. Hence such a ring provides a useful cooling to the fibre.

Coating qualification trials were carried out both on single mode and multimode fibres using the following standarised line conditions Crosshead: Maillefer Tooling : Standard Maillefer Type of cooling Rings: "air movers" Position of cooling rings: (i) AM20 - 20mm bore at 50mm from the crosshead (ii) AM40 - 40mm bore at 260mm from the crosshead (iii) AM40 - 40mm bore at 1010mm from the crosshead Total airpath : 7m Fibre input to crosshead: constant speed Fibre take up : constant tension Line tension : (200+20) g Input air flow rates through cooling rings : (i) AM20 - 35 fibreslmin (ii) AM40 - 75 fibres/min (iii) AM40 - 75 fibres/min Coated fibre o.d.: 0.85 r 0.05 mm Lump and neckdown settings: 0.03 mm.

For the single mode fibres only, the axial compression was monitored continuously for each fibre, and the fibre was paid off loose into a cardboard box. The attenuation of coated fibres was measured using the standard cut-back technique. The experimental data is shown in the attached tables I and II. Figure 6 shows the variation in fibre compression through each coating run.

From these results, the following conclusions were made (i) Fibre compression was stable throughout each coating run.

(ii) Coated fibre o.d. was well within #0,05 mm tolerance limit.

(iii) Since each coating run was done at a different time, repeatability in terms of fibre compression was fairly good.

(iv) Coating increments were small and well within acceptable limits.

(v) Coating of fairly consistent density was obtained from each coating run, for a fixed air path of 7 m.

In order to verify that an air path of 7 m was giving stable fibre compression, the axial fibre compression was measured for increasing air paths and the results are shown in Figure 7. It is evident that a minimum of 0.2 percent axial compression is obtained for an air path of 5.5 m, and that both the fibre compression and coating density reach an asymptote for air paths greater than 8 mm from the crosshead. The minimal air path was thus fixed at 7 m from the crosshead to cater for small temperature fluctuations to enable a stable coating to be achieved consistently.

Tables I and II show experimental data on single mode and multimode fibres respectively.

TABLE I Experimental data on single mode fibres Fibre No. Coating No. Coated Room Temp. Attenuation at 1,3 m Length ( C) (dBlkm) Axial Coating density (m) Before After Increment compression (Kg/m ) coating coating % 1:412004100 69NR 1960 25 0.44 0.4 0 0.254 1013.1 1:412218100 70NR 3439 20 1.03 0.92 -0.11 0.266 1012.9 1:412213100 71NR 4661 22 0.52 0.59 +0.07 0.251 1013.1 1:412036100 72NR 4136 20 0.38 0.426 +0.04 0.257 1012.7 TABLE II Experimental data of multimode fibres Fibre No. Coating No. Coated Room Temp. Attenuation at 0.85 m Length ( C) dBlkm Coating density Fibre Numerical (m) Before After Increment (Kg/m ) Aperture coating coating (NA) 2:407049100 74NR 5657 24 2.80 2.88 +0.08 1013.0 0.2 2:41128100 75NR 4219 - 3.40 3.44 +0.04 1013.0 0.2 2:409032100 76NR 6149 22 2.80 3.10 +0.30 1013.8 0.22 2:40837100 77NR 6314 21 2.80 2.92 +0.12 1012.7 0.21

Claims (10)

1. A method of applying a secondary coating to an optical fibre, in which the fibre to be coated is passed through an extrudate cross-head for the applciation of the secondary coating to the fibre, whereafter the coated fibre is passed along an air path to cool the secondary coating, and in which during the passage of said coated fibre along the air path it passes through a plurality of cooling rings each of which directs a cooling air flow onto the coated fibre so that the air directed onto the coated fibre by each said cooling ring embraces the coated fibre and thereby exerts a cooling influence on the secondary coating.

2. A method as claimed in claim 1, in which the cooling rings are three in number, with the distance between the first and the second ring being greater than the distance between the extrudate crosshead and the first ring, and with the distance between the second and the third rings being greater than the distance between the first and second rings.

3. A method as claimed in claim 1 or 2, in which each said cooling ring has its internal bore via which the fibre passes of Venturi form so that the rings each act as an air amplifier.

4. A method of applying a secondary coating to an optical fibre, substantially as described with reference to the accompanying drawings.

5. Apparatus for applying a secondary coating to an optical fibre, which includes an extrudate crosshead through which the fibre to be coated is passed for the application thereto of the secondary coating, whereafter the optical fibre is conveyed via an air path to the output of the apparatus, a plurality of cooling rings through which the coated fibre passes during its traverse of the air path, each of which cooling rings directs a cooling air flow onto the coated fibre so that the air directed onto the coated fibre embraces the fibre and thereby exerts a cooling influence on the secondary coating, a control diameter gauge located between the first and second of said cooling rings so that it controls the coating thickness, and a monitoring diameter gauge through which the coated optical fibre passes at a point near to the end of the air path.

6, Apparatus as claimed in claim 5, in which the cooling rings are three in number, with the distance between the first and the second ring greater than the distance between the extrudate crosshead and the first ring, and with the distance between the second and the third rings greater than the distance between the first and the second rings.

7. Apparatus as claimed in claim 5 or 6, in which each said cooling ring has its internal bore via which the fibre passes of Venturi form so that the rings each act as an air amplifier.

8. Apparatus as claimed in claim 5, 6 or 7, in which the air path has a length of 7.0 metres and the rate of travel of the coated fibre along the path is 100 metres/minute, in which the coating to be applied has a thickness such as to give an outside diameter of 0.85 mm, in which the first cooling ring is 50mm from the crosshead, the second cooling ring is 260 mm from the crosshead and the third cooling ring is 1110 mm from the crosshead.

9. Apparatus for applying a secondary coating to an optical fibre, substantially as described with reference to the accompanying drawings.

10. Apparatus for applying a secondary coating of a plastics material to an optical fibre, which includes an extrudate cross-head through which the fibre to be coated is passed for the application thereto of the secondary coating, whereafter the optical fibre is conveyed via an air path to the output of the apparatus, three cooling rings through which the coated fibre passes during its traverse of the air path, each of which cooling rings directs a cooling air flow onto the coated fibre so that the air directed onto the coated fibre embraces the coated fibre so as to exert a cooling influence on the secondary coating, a control diameter gauge located between the first and second of said cooling rings such that it controls the coating thickness, and a monitoring diameter gauge through which the coated optical fibre passes at a point near to the end of the air path, the cooling rings being so located along the air path that the distance between the first and the second ring is greater than the distance between the extrudate crosshead and the first ring and that the distance between the second and the third rings is greater than the distance between the first and the second rings.

10. A method, as claimed in any one of claims 1 to 9, which enables stable secondary coatings to be applied to optical fibres at a high line speed of 100 mlmin, and the said method being suitable for use in a production environment.

Amendments to the claims have been filed, and have the following effect: (b) New or textually amended claims have been filed as follows: